Higgs Boson wrote:
Amigos y amigas, this has been bugging me for years. I was taught
that after opening a faucet to its fullest, one should "back off" a
little.

WHY???

If the valve has been open fully (months/years) it may be hard to
close. Also, breaking it loose when fully open can take some of the
stem packing with it. (Stem may leak unless fully open/closed)
An example is a toilet or sink shut-off.